Since the introduction of the heart rhythm control system—first as an implantable pacemaker in the 1960's, and then as an implantable defibrillator in 1980, implantable electrical stimulating devices have been developed to treat various medical diseases and physiological ailments, such as chronic pain disorders (e.g., nerve injury, failed back syndrome, intractable facial pain, failed neck syndrome, reflex sympathetic dystrophy, thoracic outlet syndrome, and cancer), neurological disorders (e.g., intractable epilepsy and Parkinson's disease), motor disorders (e.g., spasticity, dystonia, spasmodic torticollis, athetosis, head injury, spinal cord injury, stroke, multiple sclerosis, and cerebral palsy), cardiac rhythm disorders (e.g., tachycardia and bradycardia), and psychosomatic disorders (e.g. depression and eating disorders).
Some of these implantable devices are currently being marketed. For example, implantable spinal cord stimulators are currently being used in patients to relieve pain in various parts of the body, e.g., chronic back and leg pain, cancer pain, postoperative spinal cord injury pain, and reflex sympathetic dystrophy pain. The implantation process involves placing leads in the epidural space of the spinal canal in a location that corresponds to the patient's zone of pain. A pulse generator is then implanted in the lower anterior abdominal wall and then connected to electrodes on the leads via an extension that is percutaneously routed from the pulse generator to the leads. Once the system is fully implanted, the pulse generator can then be operated to provide low-voltage electrical stimulation of the spinal cord via the leads.
Another means for managing pain involves implanting micro-current electrical neuromuscular stimulators (MENS) within a patient in the area of the perceived pain. These devices use a very low current (typically 1-100 μA) and operate on a cellular level to speed the healing process, thereby reducing pain. These devices have been specifically used to treat arthritic conditions, sports injuries, low back pain, carpal tunnel, tennis elbow, migraines and other disorders.
A stimulation system similar to the spinal cord stimulator described above is currently being used to treat Parkinson's Disease. In this application, a lead is surgically implanted into the patient's brain adjacent the subthalamic nucleus (STN) or globus pallidus internal (GPi), which control the involuntary movement symptomatic of Parkinson's Disease. A pulse generator is implanted in the patient's chest near the collarbone, and then connected to electrodes on the lead via an extension that percutaneously runs from the pulse generator to the lead. The pulse generator can then be operated to electrically stimulate the effected regions of the brain in order to block the signals that cause the disabling motor symptoms.
Pacemakers are used to alter the heart rate of a patient. A pacemaker, like the previously described devices, includes a pulse generator and leads. The pulse generator is implanted in a sub-dermal pocket in the patient chest, while the leads are inserted into a vein underneath the collar bone and threaded into the heart. Depending on the specific medical problem, a pacemaker can replace the S-A node signals that are delayed or get lost in the electrical pathway between the upper and lower heart. A pacemaker can also maintain a normal timing sequence between the upper and lower heart, and make sure that the critical lower chambers of the heart contract at an adequate rate. A pacemaker can pace a single chamber or two chambers of the right side of the heart, or even synchronize the two ventricles for optimizing the heart pumping capability.
Other electrical stimulating devices might include peripheral nerve stimulators, medical delivery or activation systems, pumps for supporting a failing heart, controlling incontinence, or controlling the opening of a body passage, such as in a shunt for treating hydrocephalus.
All of the above-described devices provide power to the pulse generator in one of two ways: installing a battery within the pulse generator or transmitting wireless energy to the generator, e.g., by wirelessly transmitting power from an external transmitter via radio frequency (RF) telemetry to an internal sub-dermal receiver hard-wired to the pulse generator. In some cases, the pulse generators may be wirelessly controlled by an external command, for example, to obtain data, and/or to activate or otherwise control the pulse generator. For those devices that use a battery, the size of the battery required to support the operational life of the device prevents the pulse generator from being located adjacent the stimulated region. As a result, the pulse generator is usually located in the body remote from electrodes, and a lead must be used to connect the generator and electrodes. For those devices that wirelessly supply power or data to the generator, RF energy may only penetrate a few millimeters into a body, because of the body's dielectric nature. Thus, RF energy may not be able to provide power to, or otherwise communicate with, an implant that is located deep within the body.
Regardless of the means used to supply power to the pulse generator, it is sometimes possible to feel the pulse generator under the skin and visually notice a slight deformity of the skin region that covers the generator. In addition, the requirement of a lead adds complexity to the medical procedure and increases the risk of infection, and in the case of pacemakers and spinal cord stimulators, increases the risk of damaging heart valves that are crossed by the lead or the spinal cord over which the lead is routed. In some cases, the electrodes may become destabilized due to the forces applied on them by the lead. In addition, the leads are typically composed of metal formed as a coil in which electrical current may be adversely induced in the presence of a strong magnetic field, such as that created during Magnetic Resonance Imaging (MRI). As a result, those patients in which these leads are implanted cannot undergo an MRI.